add missing classname to "Install Prometheus" button

ee/app/views/projects/clusters/_health.html.haml

@@ -17,5 +17,5 @@
     .settings-content
       %p= s_("ClusterIntegration|In order to show the health of the cluster, we'll need to provision your cluster with Prometheus to collect the required data.")
 
-      %a.btn{ href: '#cluster-applications' }
+      %a.btn.btn-default{ href: '#cluster-applications' }
         = s_('ClusterIntegration|Install Prometheus')